4+2 rule

Hello all:
I am wondering whether the 4+2 rule actually applies to oligo conditions
that we frequently use (for PCR, sequencing, etc). Specifically, "it has
been said" that the 4+2 rule applies to Tm's under 1M salt conditions,
which would essentially invalidate it (Tm-16.6) for most applications.
The noly ref to a reference that I have found was to an old NAR paper that
didn't discuss this at all.
So...
Who knows the origin of the 4+2 rule, and/or what annealing conditions it
was designed to apply to?
thanks!
bob darnell
--
Bob Darnell
Rockefeller University
New York, NY